# Waiting and retrying

Source: https://libtmux.org/en/py/stable/topics/waiting-and-retry/

> Polling a condition instead of guessing a sleep, and tmux's own wait-for signal channel as the alternative to polling.

After sending input or starting a process, wait for the state your next step
requires. [Pane
interaction](https://libtmux.org/en/py/stable/topics/pane-interaction/#waiting-for-something-to-finish) covers
waiting for screen text. This page covers arbitrary conditions and tmux's named
[`wait-for`](https://libtmux.org/en/tmux/latest/manual/wait-for/) signal channels.

## Polling a condition

Polling checks a condition repeatedly until it succeeds or a deadline expires.
Set a deadline and choose an interval that limits unnecessary tmux commands.

**Helper:** `libtmux.test.retry_until(fn, seconds=, interval=)`

**Where it lives:** [`src/libtmux/test/`](<https://github.com/tmux-python/libtmux/tree/38e368c11117fb4aeb2f082d552cd4f210eae06a/src/libtmux/test>) in the main package; raises
[`WaitTimeout`](<https://libtmux.org/en/py/stable/reference/libtmux-exc-waittimeout/>).

### Examples

```python
def is_window_up(pane, name):
    return any(w.window_name == name for w in pane.window.session.windows)

libtmux.test.retry_until(lambda: is_window_up(pane, "build"), seconds=5.0)
```

## tmux's own [wait-for](https://libtmux.org/en/tmux/latest/manual/wait-for/) channel

Use [`tmux wait-for -S <channel>`](https://libtmux.org/en/tmux/latest/manual/wait-for/) to signal and [`tmux wait-for <channel>`](https://libtmux.org/en/tmux/latest/manual/wait-for/) to
block until signalled. This avoids repeated screen captures when the command can
announce its own completion:

Signal a channel with [`Server.wait_for`](<https://libtmux.org/en/py/stable/reference/libtmux-server-wait_for/>) and `set_flag=True`.
Call the same method without that flag to wait for the signal.

```python
server.new_session(session_name="work")
server.wait_for("built", set_flag=True)  # signal
server.wait_for("built")                 # block until signalled
```

tmux remembers a signal sent before a waiter starts. The next wait on that
channel returns immediately, so completion is not lost when the command finishes
first.

A raw [`wait-for`](https://libtmux.org/en/tmux/latest/manual/wait-for/) client can exit zero when the server dies, as well as when the
channel is signalled. Verify server liveness when a lost server must be treated
as a failed task.

Use a channel name specific to the task. A remembered signal can otherwise
satisfy an unrelated later wait.

<details>
<summary>tmux manual and source</summary>

The tmux manual describes [completion channels](https://github.com/tmux/tmux/blob/94796f6b1182507efac8a272fc309a79e22e58a5/tmux.1#L8715).
The [channel implementation](https://github.com/tmux/tmux/blob/94796f6b1182507efac8a272fc309a79e22e58a5/cmd-wait-for.c#L388)
retains an early signal until a waiter consumes it. Use a fresh channel name
for each operation.

</details>